The present invention relates to improved combustion processes and more particularly to improved methods and apparatus for calcining carbonaceous material such as petroleum coke.
Carbonaceous materials such as "green" petroleum coke are typically calcined in a rotary kiln by introducing these materials into the upper end of a slightly inclined kiln and heating the interior thereof to a temperature of approximately 2200.degree.-2500.degree. F. One technique for heating such a kiln is to fire an end burner disposed at the product outlet end or lower end of the kiln and directing a flame longitudinally through the kiln in the direction generally opposed to that of the product being calcined. This application of heat is effective to evolve or drive off volatile materials from the green carbonaceous material thereby increasing the density of such materials being calcined. As these gases are evolved from the green carbonaceous material, the heat within the kiln is effective to cause a combustion of such volatile gases which in turn releases heat to the kiln interior and enables the firing rate of the end burner to be reduced below a relatively high firing rate. Frequently, combustion of volatiles in the kiln will supply a majority of the heat required to calcine a green carbonaceous material such as petroleum coke. Typically, during calcination, at least 99% of the volatiles of a green carbonaceous material are evolved therefrom and are combusted as virtually complete volatilization is required in order to produce a calcined product of suitable quality. With regard to petroleum coke, calcination will typically be effective to increase the density thereof from approximately 1.6 to about 2.0 g/cm.sup.3. This enables the resulting petroleum coke to be utilized for several purposes including use as a fuel.
In order to utilize the heat available in volatiles evolved during the calcining of carbonaceous material, it is known to introduce air in controlled quantities through a fan or blower mounted for revolution with the kiln itself. Such a system is illustrated in U.S. Pat. No. 2,813,822 which shows a kiln mounted blower adapted to supply controlled air flows through a plurality of tuyeres into the interior of the kiln. This forced introduction of air into the kiln is effective to improve the combustion of volatile gases evolved during the calcining of a material such as petroleum coke. More recently, it has been proposed in U.S. Pat. No. 3,888,621 to control the air supplied by a kiln mounted blower in response to interior kiln conditions. In this reference, apparatus for optically detecting smoke conditions within a kiln is provided such that the flow of air into the kiln is controlled thereby enabling an improved combustion of volatile materials while avoiding significant combustion of the carbonaceous material, i.e. petroleum coke. In addition, it is also known to adjust the residence time of a carbonaceous material in different portions of a rotary kiln so that combustion of volatile materials evolved from such carbonaceous materials may be improved. One such technique along these lines is illustrated in U.S. Pat. No. 3,966,560. Finally, it has also been proposed to optimize kiln temperatures by establishing reference temperatures at various locations of the kiln and adjusting the supply of air and kiln revolution rate, etc. in order to optimize calcination of carbonaceous materials as is described in U.S. Pat. No. 4,022,569.
Notwithstanding the foregoing improvements in processes and equipment for calcining carbonaceous material, it is frequently desirable to increase the rate at which a carbonaceous material can be adequately calcined in a rotary kiln of a predetermined size without incurring significant structural modifications and capital additions. Simply increasing the feed rate of carbonaceous material to the kiln will not result in a greater throughput of calcined material as volatiles in the material will not be volatilized sufficiently to increase density to predetermined levels. Consequently, increasing the material feed rate alone merely results in partial, incomplete volatilization which does not yield an acceptably calcined product. Increasing kiln speed alone will reduce the residence time of carbonaceous material in the kiln and will also result in incomplete volatilization and ineffective calcination. Thus, the prior art has exhibited a clear need for improving the throughput rates of rotary kilns utilized to effectively calcine carbonaceous material without extensive additional structural modifications and attendant costs.